1. Field of the Invention
The present invention relates to a casting aluminum alloy and a heat treatment method thereof. More specifically, the present invention relates to an aluminum alloy suitably used for a member for which both of an excellent high cycle fatigue strength and an excellent thermal fatigue strength are required, to a casting made of the alloy, and a manufacturing method of the casting. Moreover, the present invention relates to an internal combustion engine cylinder head composed of the aluminum alloy and manufactured by the manufacturing method of the casting.
2. Description of the Related Art
As a casting alloy that has a complicated shape, for which excellent mechanical properties are required, heretofore, aluminum alloy castings have been used, which are of Al—Cu—Si series defined as AC2A, AC2B and AC4B in JIS H 5202, and of Al—Mg—Si series defined as AC4C and AC4CH therein. As castings of these alloys, there are a cylinder head, a cylinder block and the like for an internal combustion engine.
In these castings, as disclosed in Japanese Patent Laid-Open Publication No. 2006-169594, it is frequent that casting bodies are used, which have been subjected to T6 treatment (aging treatment at a tempering temperature, at which the maximum strength is obtained, after solution heat/quenching treatment) or T7 treatment (treatment for ensuring dimensional stability by overaging after solution heat/quenching treatment) for the purpose of enhancing strength and ductility.
However, in such a conventional internal combustion engine cylinder head, as engine power has been increased and the cylinder head has been thinned aiming at weight reduction of a vehicle body in recent years, a cyclic stress has tended to be increased. In addition, the cylinder head has had a structure in which a high residual stress generated at the time of the T6 or T7 heat treatment is locally concentrated. Accordingly, in the aluminum alloy casting as described above, it cannot be said that elongation thereof as alternative properties of the high cycle fatigue strength and the thermal fatigue strength is sufficient, and there has been a problem of an increased possibility of a fatigue crack occurrence. Such fatigue cracks may occur from stress-concentrated portions of a top deck and water jacket of the cylinder head, and from a high-temperature portion of an inter-valve portion in a combustion chamber.
The present invention has been made focusing attention on the above-described problem in the conventional aluminum alloy casting. It is an object of the present invention to provide a casting aluminum alloy that is excellent in elongation as the alternative properties of the thermal fatigue strength and the high cycle fatigue strength and is suitably usable for a casting for which both of the excellent high cycle fatigue strength and the excellent thermal fatigue strength are required, for example, an internal combustion engine cylinder head, to provide a casting made of the aluminum alloy, to provide a manufacturing method of the casting, and further, to provide an internal combustion engine cylinder head composed of the aluminum alloy casting, and to provide an internal combustion engine cylinder head manufactured by the manufacturing method of the casting.